Image display systems that employ image projection techniques, such as spatial light modulators (SLMs) using arrays of digital micromirror devices (DMDs), deformable mirrors, and liquid crystal on silicon, can suffer from a degraded contrast ratio (a ratio of brightest white to darkest black producible) due to reflections of a light used to project the images from parts of the arrays not intended to reflect light. Light can scatter from support structures, apertures, vias, and so forth from the array of light modulators. The scattered light can effectively reduce the darkness of the darkest black, therefore reducing the contrast ratio. For example, in an SLM using DMD technology, light can scatter from electrode structures, micromirror support structures, mirror vias, and so on.
One technique to reduce reflection is to place an antireflective coating on metal structures in the array of light modulators. Since the metal structures can be a significant source of unintended reflection, the use of the antireflective coating can significantly improve the contrast ratio. For example, uncoated aluminum can have a reflectivity as high as 92 percent while coated aluminum can have a reflectivity of approximately two to three percent.
One disadvantage of the prior art is that the application of the antireflective coating on the metal structures does not address the unintended reflection from other portions of the array of light modulators, such as the interlayer dielectrics, the substrate material, metal structures buried under dielectric layers, and so forth.
A second disadvantage of the prior art is that by not addressing the light reflecting off the non-metallic structures, it is possible to have an undesired change in the color point by using different layer thicknesses. Changes in the thicknesses of the interlayer dielectric layers can have an especially pronounced effect upon the color point. Changing the color point can result in an undesired colorcast to the images being displayed.